Coaxial transmission line slot filter with absorptive matrix

ABSTRACT

A filter is provided and includes potting material formed into a body defining a through-hole. The body includes first and second opposing faces and a sidewall extending between the first and second opposing faces. The sidewall is formed to define first and second openings at opposite ends of the through-hole, first angles at an interface between the sidewall and the first face and second angles, which complement the first angles, at an interface between the sidewall and the second face.

DOMESTIC BENEFIT/NATIONAL STAGE INFORMATION

This application is a divisional application of U.S application Ser. No.13/837,606, which was filed on Mar. 15, 2013. The entire disclosures ofU.S application Ser. No. 13/837,606 are incorporated herein byreference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under Contract No.:W911NF-10-1-0324 awarded by Army Research Office (ARO). The Governmenthas certain rights in this invention.

BACKGROUND

The present invention relates to a coaxial transmission line filter and,more particularly, to a coaxial transmission line slot filter with anabsorptive matrix.

Microwave circuits often rely on filters in order to isolate signalsfrom noise sources, or from other portions of the circuit. These filtersare often bulky and require extra hardware in addition to the wiring andstructures already in place for the circuits in use. In some cases, thefilters are not widely tunable in either frequency cutoff orattenuation. Further, the filters may or may not be low-temperaturecompatible. Finally, the filter cost may be high due to the specialhardware required to implement the design.

SUMMARY

According to one embodiment of the present invention, a filter isprovided and includes potting material formed into a body defining athrough-hole. The body includes first and second opposing faces and asidewall extending between the first and second opposing faces. Thesidewall is formed to define first and second openings at opposite endsof the through-hole, first angles at an interface between the sidewalland the first face and second angles, which complement the first angles,at an interface between the sidewall and the second face.

According to another embodiment of the invention, a filter is providedfor a coaxial cable including an outer sheath having one or more slitsformed therein exposing a dielectric spacer between the outer sheath andan inner conductor. The filter includes potting material formed into abody about the outer sheath. The potting material contacts the exposeddielectric spacer via the one or more slits. The body defines athrough-hole through which the coaxial cable extends and includes firstand second opposing faces on opposite sides of the coaxial cable and asidewall extending between the first and second opposing faces. Thesidewall is formed to define first and second openings for the coaxialcable, first angles at an interface between the sidewall and the firstface and second angles, which complement the first angles, at aninterface between the sidewall and the second face.

According to another embodiment of the invention, a filtered coaxialcable is provided and includes an inner conductor, an outer sheathhaving one or more slits formed therein, a dielectric spacer between theinner conductor and the outer sheath, portions of the dielectric spacerbeing exposed by the one or more slits and potting material formed intoa body about the outer sheath such that the potting material contactsthe portions of the dielectric spacer exposed by the one or more slits.

According to another embodiment of the invention, a method of forming afilter is provided for a coaxial cable including an outer sheath havingone or more slits formed therein exposing a dielectric spacer betweenthe outer sheath and an inner conductor. The method includes forming amold with an opening, attaching the coaxial cable to the mold such thatthe coaxial cable extends through the opening and pouring pottingmaterial into the opening and curing the potting material. The formingof the mold includes shaping the opening such that the potting materialis formable into a body about the outer sheath such that the pottingmaterial contacts the exposed dielectric spacer via the one or moreslits.

According to yet another embodiment of the invention, a method offorming a filtered coaxial cable including an inner conductor, an outersheath and a dielectric spacer disposed between the inner conductor andthe outer sheath is provided. The method includes forming one or moreslits in the outer sheath, assembling a mold with an opening, disposingthe coaxial cable on the mold such that the coaxial cable is suspendedin the opening, pouring potting material into the opening and curing thepotting material such that the cured potting material contacts portionsof the dielectric spacer exposed by the one or more slits and removingthe coaxial cable from the mold.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 is a plan view of a filter in accordance with embodiments;

FIG. 2 is a side view of the filter of FIG. 1;

FIG. 3 is an enlarged partial side view of the filter of FIG. 1;

FIG. 4 is a perspective view of a tool use to form the filter of FIG. 1;

FIG. 5 is a perspective view of the tool of FIG. 3 in an initial stageof use;

FIG. 6 is an enlarged partial side view of the filter of FIG. 1 inaccordance with further embodiments;

FIG. 7 is a graphical depiction of a performance of the filter of FIG.1; and

FIG. 8 is a graphical depiction of a performance of the filter of FIG.1.

DETAILED DESCRIPTION

The description provided herein relates to a filter (e.g., a radiofrequency (RF) filter) that is based on a standard coaxial cablestructure. The cable's outer sheath is slotted longitudinally to exposea dielectric spacer between the outer sheath and the center conductor,and the resultant opening is potted in epoxy in which ferromagneticparticles are embedded. This filter is relatively inexpensive in that ituses only the existing cable used in the apparatus plus some inexpensiveepoxy, tunable by an adjustment of the ferromagnetic particle contentand/or the dimensions and number of slots, is cryogenic compatible andis compact as compared to other solutions. The filter removes highfrequency components from the signal line yet presents a nearly constantimpedance to lower frequency signals, which are passed by the device.The rejection band characteristics for this filter have the desirableproperty that attenuation increases with increasing frequency with noreduction in attenuation up to (and possibly above) about 65 GHz. Thisis an unusual and desirable property especially in view of the fact thatmost other low-pass filters show increasing attenuation up to somefrequency, but then above that frequency the attenuation actuallydecreases.

With reference to FIGS. 1-3, a filter 10 is provided for use in variousapplications such as those associated with a coaxial cable 20. Thecoaxial cable 20 includes a conductive outer sheath 21, an innerconductor 22 and a dielectric spacer 23 operably disposed between theouter sheath 21 and the inner conductor 22 to insulate the outer sheath21 and the inner conductor 22 from one another. The outer sheath 21 hasone or more slits 24 formed therein, which expose portions 230 of thedielectric spacer 23. The slits 24 do not extend around an entirecircumference of the outer sheath 21 such that the outer sheath 21 is acontinuous element. The slits 24 may have similar shapes and sizes orvaried shapes and sizes. The coaxial cable 20 may further includeconnectors 25 at opposite axial ends thereof that will generally havelarger diameters than the outer sheath 21. The filter 10 furtherincludes potting material 30 that is formed into a body 31 disposedabout the outer sheath 21 such that the potting material 30 contacts theexposed portions 230 of the dielectric spacer 23 via the slits 24 (seeFIG. 3 and, for further embodiments, FIG. 6).

The body 31 is formed to define a through-hole 310 through which thecoaxial cable 20 extends in a longitudinal dimension. The body 31includes first and second opposing faces 311 and 312 on opposite sidesof the coaxial cable 20 and a sidewall 313 that extends between thefirst and second opposing faces 311, 312. The sidewall 313 is formed todefine first and second openings 314 and 315 for the coaxial cable 20 toenter and exit the through-hole 310, first angles 316 and second angles317. The first angles 316 are defined at an interface between thesidewall 313 and the first face 311. The second angles 317 complementthe first angles 316 and are defined at an interface between thesidewall 313 and the second face 312.

In accordance with embodiments, the first and second faces 311 and 312may be substantially planar and, in accordance with further embodiments,the first and second faces 311 and 312 may be substantially parallelwith one another. In general, however, it is not necessary for the firstand second faces 311 and 312 to have any particular characteristicplanarity and need not be parallel with one another. As shown in FIG. 1,the first and second faces 311 and 312 may be substantially oblong withrounded or semi-circular ends and parallel sides. The first and secondangles 316 and 317 are complementary with one another at correspondingportions of the first and second faces 311 and 312. That is, the firstangle 316 formed at a first given point of a periphery of the first face311 complements the second angle formed at a second point of theperiphery of the second face 312 that corresponds (i.e., in terms of itsdistance to the ends of the body 31) to the first given point. In somecases, the first and second angles 316 and 317 will be right angles.However, embodiments could exist in which the first angles 316 areobtuse and the second angles 317 are correspondingly acute and viceversa.

The potting material 30 may include an absorptive epoxy material 300(see FIG. 5) and ferromagnetic particles. In accordance withembodiments, the absorptive epoxy includes Eccosorb™.

A method of forming the filter 10 will now be explained with referenceto FIGS. 4 and 5. Initially, as shown in FIG. 4, a mold 40 is made froma top piece 41 and a bottom piece 42. Both the top piece 41 and thebottom piece 42 may be formed from Teflon™ or another similar non-stickmaterial. The top piece 41 has opening 410 cut completely through it.This opening 410 may be rectangular or, more specifically, rectangularwith rounded or semi-circular ends. Narrow slots 411 are cut in each endof the opening 410 that are at least as wide as the diameter of thecoaxial cable 20 (the diameter of the coaxial cable 20 may be about0.085 inches). The bottom piece 42 is formed as a substantially flatbody of Teflon™ or another similar non-stick material. The top piece 41is affixed (i.e., fastened or screwed) to the bottom piece 42. With thisconstruction, the opening 410 is bordered by the cutout sidewalls of thetop piece 41 and the top surface of the bottom piece 42. The shape ofthe cutout sidewalls of the top piece and the flatness (or lack thereof)of the surface of the bottom piece 42 will be determinative of the shapeof the body 31 as described above.

The use of Teflon™ (or another similar non-stick material) facilitatesthe removal of the filter 10 form the mold 40 once the potting material30 is cured as will be described below. Since the cured potting material30 does not stick to the Teflon™, it should be possible to simply slidethe filter 10 and the coaxial cable 20 out and away from the mold 40.Where the first and second angles 316 and 317 are formed as rightangles, this sliding should be conducted along an axis oriented normallyto the top surface of the bottom piece 42 (i.e., the “removal direction”of FIG. 2). However, in the case of the embodiments in which the firstangles 316 are obtuse and the second angles 317 are correspondinglyacute and vice versa, the sliding may be conducted at an angle withrespect to the top surface of the bottom piece 42 (i.e., the “angledremoval direction” of FIG. 2).

As shown in FIG. 5, the coaxial cable 20 with the slits 24 (see FIG. 1)formed in the outer sheath 21 is inserted into the narrow slots 411 inthe top piece 41 such that the coaxial cable 20 is suspended inside theopening 410. In accordance with embodiments, the height of the top piece41 should be defined should such that the suspended cable issubstantially centered in the opening 410. Sealant (such as anon-magnetic epoxy) 50 is then applied around the ends of the coaxialcable 20 where the coaxial cable 20 exits the opening 410 through thenarrow slots 411.

The absorptive epoxy material 300 is then poured into the opening 410and thereby fully encapsulates the coaxial cable 20 such that itcontacts the dielectric spacer 23 via the slits 24. The absorptive epoxymaterial 300 is allowed to cure until solid. At this point, the filter10 and the coaxial cable 20 are removed from the mold 40. Removal may beaccomplished by simply pulling the filter 10 and the coaxial cable 20outwardly or by separating the top and bottom pieces 41 and 42 of themold 40 and pressing out the filter 10 from the opening 410 in eitherthe removal direction of FIG. 2 or the angled removal direction of FIG.2.

In accordance with embodiments, the slits 24 may be 1 mm in width andmay have varying lengths. By adjusting the slit 24 lengths and thenumber of slits 24, properties of the filter 10, such as attenuationversus frequency, can be set to desired values. The slit 24 geometrydetermines the amount and frequency of radiation that escapes into thepotting material 30. This radiation is then absorbed by theepoxy/ferromagnetic particle combination of the potting material 30 andleads to the observed behavior of the filter 10.

As shown in FIG. 3, the formation of the one or more slits 24 mayinclude the removal of only material of the outer sheath 21. In thesecases, none of the dielectric spacer 23 is removed. However, inaccordance with further embodiments and, with reference to FIG. 6, someof the dielectric spacer 23 may be removed along with the material ofthe outer sheath 21. As shown in FIG. 6, in these cases, a portion ofthe potting material 30 infiltrates the spacer that would otherwise beoccupied by the removed dielectric spacer 23. In accordance with stillfurther embodiments and, as shown in FIG. 6, once the filter 10 and thecoaxial cable 20 are removed from the mold 40, an outer metalliccylinder 60 may be disposed about the filter 10 to retain the pottingmaterial 30 therein.

With reference to FIGS. 7 and 8, graphical depictions of performances ofthe filter 10 are illustrated. In FIG. 7, the filter 10 has 2 slits 24,which are 1 cm long and 1 mm wide. This filter 10 is operable for 3 dBat 12 GHz and provides a maximum attenuation of 5 dB 65 GHz. In FIG. 8,the filter 10 has 4 slits 24, that are 2.6 cm long and 1 mm wide. Thisfilter 10 is operable for 3 dB at 6-7 GHz and provides a maximumattenuation of 15 dB 65 GHz.

It is to be understood that a parameter affecting performance of thefilter 10 is whether or not any of the dielectric spacer 23 is removedwhen the slits 24 are formed. As noted above and, in accordance withembodiments, most if not all of the dielectric spacer 23 remains andonly sections of the outer sheath 21 are removed to form the slits 24(see FIG. 3). This geometry minimizes the impedance mismatch between thesection of the coaxial cable 20 that is associated with the filter 10and the other non-filtered sections of the coaxial cable 20. Inaccordance with alternative embodiments, however, at least some of thedielectric space 23 is removed in the formation of the slits 24. Theseembodiments may be particularly useful with a fixed size slit 24 where arelatively large impedance mismatch is acceptable (see FIG. 6).

In accordance with further aspects and, with reference to FIG. 1, acircuit 100 is provided. The circuit employs the filtered coaxial cable20 as described above with the filtered coaxial cable 20 being disposedin a low-temperature measurement apparatus 110 at various periods in thecooling stages. In this way, the filter 10 serves to attenuate unwantedhigh frequency noise signals. In accordance with embodiments, a designcorner frequency may be in the range of about 2-20 GHz, and theattenuation would be adjusted by choice of Eccosorb™ materials, slit 24lengths and widths and the number of slits 24 in order to achieve thedesired reduction in noise at any given frequency.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of onemore other features, integers, steps, operations, element components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

While the preferred embodiment to the invention had been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

What is claimed is:
 1. A method of forming a filter for a coaxial cableincluding an outer sheath having one or more slits formed thereinexposing a dielectric spacer between the outer sheath and an innerconductor, the method comprising: forming a mold with an opening;attaching the coaxial cable to the mold such that the coaxial cableextends through the opening; and pouring potting material into theopening and curing the potting material, the forming of the moldcomprises shaping the opening such that the potting material is formableinto a body about the outer sheath such that the potting materialcontacts the exposed dielectric spacer via the one or more slits.
 2. Themethod according to claim 1, wherein the potting material comprises anabsorptive epoxy and ferromagnetic particles.
 3. The method according toclaim 1, wherein the mold comprises non-stick material.
 4. The methodaccording to claim 1, wherein the forming of the mold comprises shapingthe opening such that the body is outwardly slideable.
 5. A method offorming a filtered coaxial cable including an inner conductor, an outersheath and a dielectric spacer disposed between the inner conductor andthe outer sheath, the method comprising: forming one or more slits inthe outer sheath; assembling a mold with an opening; disposing thecoaxial cable on the mold such that the coaxial cable is suspended inthe opening; pouring potting material into the opening and curing thepotting material such that the cured potting material contacts portionsof the dielectric spacer exposed by the one or more slits; and removingthe coaxial cable from the mold.
 6. The method according to claim 5,wherein the forming of the one or more slits comprises removing littleto none of the dielectric spacer.
 7. The method according to claim 5,wherein the forming of the one or more slits comprises removing at leastsome of the dielectric spacer.
 8. The method according to claim 5,wherein the pouring of the potting material comprises pouring absorptiveepoxy with ferromagnetic particles.
 9. The method according to claim 5,further comprising disposing an outer metallic cylinder about thefiltered coaxial cable.
 10. A method of assembling a filter, the methodcomprising: forming potting material into a body defining athrough-hole, the body comprising first and second opposing faces and asidewall extending between the first and second opposing faces; definingin the sidewall: first and second openings at opposite ends of thethrough-hole, first angles at an interface between the sidewall and thefirst face, and second angles, which complement the first angles, at aninterface between the sidewall and the second face.